For short generic text, use staff or system text. The difference between these two texts is whether you want it to apply to a single staff, or to the whole system. This distinction can be clear when extracting parts.

For an overview about the new features, see What's New in MuseScore 2, MuseScore 2.0 Release Notes and Changes for MuseScore 2.0.
Documentation of new features are available in the chapter they belong to logically (except the one that is referring to upgrading from 1.3 to 2.0) , but for users coming from 1.x here's a collection of links to be able to see at a glance what can be done now...